1. Field of the Invention
The present invention relates to a method of drilling, casing and/or completing a borehole, and in particular to a method of drilling, casing and/or cladding a borehole. The invention also provides apparatus for completing a borehole. It will be understood that use of the term “borehole” herein is a reference to a bore that has been drilled into a formation to allow the recovery of hydrocarbons (or other fluids) therefrom as is conventional in the art.
2. Description of the Related Art
When a borehole has been drilled into a formation to facilitate, for example, the recovery of hydrocarbons from a well or reservoir, the formation surrounding the borehole is typically lined with a casing. Casing is installed to prevent the formation around the borehole from collapsing, and additionally to prevent unwanted fluids flowing from the surrounding formation into the borehole, and similarly, to prevent fluids from within the borehole escaping into the surrounding formation.
Referring to FIG. 1 there is shown a conventional borehole 10 that has been drilled into a formation 12. It should be noted that FIG. 1 is not to scale. Borehole 10 is drilled with a relatively large diameter at or near surface 14, and it will be appreciated that surface 14 could be below sea level.
A relatively large outer diameter (OD) casing 16 is then inserted into borehole 10 and cemented into place using cement 18 in a conventional manner. The cementing process typically involves filling an annulus between the casing 16 and the surrounding formation 12 with the cement 18 by pumping the cement 18 into the casing 16 followed by a rubber or other plug (not shown) on top of the cement 18. Thereafter, drilling fluid or the like is pumped down the casing 16 above the plug and the cement 18 is pushed out of the bottom of the casing 16 and up into the annulus between the casing 16 and the formation 12, as shown in FIG. 1. Pumping of drilling fluid (and thus the cement 18) is stopped when the plug reaches the bottom of the casing 16 and the borehole 10 must be left, typically for several hours, whilst the cement sets.
Thereafter, a smaller diameter borehole 20 is drilled through the cement 18 into the formation 12 and a subsequent casing 22 of smaller OD than the casing 16 is passed through the casing 16 above and the borehole 20. The diameter of the drill bit that is used to drill borehole 20 is typically smaller than the drill bit used to drill borehole 10, and is typically smaller than an inside diameter (ID) of the casing 16. Casing 22 is then cemented into place using cement 24 in the conventional manner, as described above. The OD of the subsequent casing 22 is limited by the inner diameter of the preceding casing 16. The cement 24 is then left for a further period of several hours to set.
A smaller diameter borehole 26 is then drilled into the cement 24 and into the formation 12, and another casing 28 is then passed through borehole 26 and the casing 22 above. As before, the diameter of the drill bit used to drill borehole 26 is typically smaller than the drill bit used to drill boreholes 10, 20, and typically smaller than the ID of the casing 22. Casing 28 is then cemented into place using cement 30 in the conventional manner described above. The cement 30 is typically left for a further period of several hours to set. The ID of the casing 22 thus limits the OD of casing 28.
Finally, a smaller diameter borehole 32 is drilled into cement 30 and into formation 12, and another casing 34 of smaller OD than casing 28 is passed through casing 28. Again, the diameter of the drill bit used to drill borehole 32 is smaller than those used to drill the preceding boreholes 10, 20, 26, and smaller than the ID of casing 28. Cement 36 is then used to secure casing 34 within borehole 32 using the conventional manner described above. The cement 36 is typically left for a further period of several hours to set.
Thus, the casings 16, 22, 28, 34 are cascaded with the diameters of the successive portions of casing reducing as the depth of the borehole 10, 20, 26, 32 increases. It will be appreciated that the depth of the borehole 10, 20, 26, 32 may be in the order of several kilometers and the example shown in FIG. 1 is representative only.
The successive reduction in diameter of casing results in a casing with a relatively small ID near the bottom of the borehole 32 at or near a formation payzone. The narrow ID could limit the amount of hydrocarbons that can be recovered. In addition, the relatively large diameter borehole 10 at the top of the well involves increased costs due to the large drill bits required, heavy equipment for handling the larger casing, and increased volumes of drill fluid that are required.
Once the casing portions 16, 22, 28, 34 have been cemented into place, the borehole is then “completed”. This involves installing a completion string 38 within the IDs of the casing portions 16, 22, 28, 34. The OD of the completion string 38 is thus limited by the ID of the lowermost casing 34, which in turn is limited by the IDs of the casings 16, 22, 28 above, and this can limit the amount of hydrocarbons that can be recovered from a reservoir 40. The completion string 38 is typically of a corrosion resistant material as corrosive chemicals in the formation 12 and/or the reservoir 40 such as H2S can be mixed with the hydrocarbons from the reservoir 40 flowing up through the string 38 to the surface 14. The flow of hydrocarbons is indicated schematically by arrows 42 in FIG. 1.
A packer 44 or the like is used at or near a lower end of the lowermost casing 34 to isolate the annulus and thus prevent hydrocarbons from flowing up it. Also, a safety valve (not shown) is typically located in the completion string 38 at or near an upper end thereof, and is used to prevent the flow of hydrocarbons to the surface in the event of an emergency, as is known in the art. The completion string 38 may also contain various flow control devices to control the flow of hydrocarbons, and downhole sensing and measuring apparatus to monitor the flow rate, temperature and other parameters of the produced fluids.